1. Field of the Invention
The present invention relates to a piezoelectric filter, a duplexer including the piezoelectric filter, a composite piezoelectric resonator, a communication device and a method for adjusting the frequency of a piezoelectric filter.
2. Description of the Related Art
Some types of piezoelectric filter include a plurality of energy-trap type, e.g. thickness longitudinal vibration type, piezoelectric resonators as vibration portions. With respect to such a piezoelectric filter, in general, a frequency band for filtering operation is determined by individually adjusting the thickness of each vibration portion.
Three methods are known for adjusting the thickness. The methods include (1) a method in which a film is added to the electrode of each vibration portion by evaporation, sputtering or other process while portions other than the required electrode are covered with a resist through the use of photolithography in order that the film thickness is selectively increased, (2) a method in which etching is performed to decrease the film thicknesses of the electrode and the like while the course of an ion beam is adjusted (refer to Patent literature 1) and (3) a method in which the frequency is adjusted by forming a plurality of fine holes in an electrode through the use of a laser (refer to Patent literature 2).
Patent literature 1 is Japanese Unexamined Patent Application Publication No. 2001-196882 and Patent literature 2 is Japanese Unexamined Patent Application Publication No. 10-154916.
With respect to the above-described method (1), the film thickness precision in the film formation with a general film formation apparatus is on the order of a low ±1%. In this case, not only the variations in targeted frequency are on the order of ±1%, but also, for example, the difference between the respective resonant frequencies of a series resonator and a parallel resonator constituting a ladder filter varies on the order of ±1% relative to a desired setting in the same wafer for manufacturing a filter.
With respect to the above-described methods (2) and (3), the frequency can simply be adjusted on an element basis. Consequently, the adjustment can be performed while the frequency characteristic is measured on an element basis, so that highly accurate adjustment can be performed. However, many adjustment steps are required for the adjustment, and thereby, the operation efficiency is significantly reduced.